Gaia TGAS search for Large Magellanic Cloud runaway supergiant stars:Candidate hypervelocity star discovery, and the nature of R71

TL;DR

This study uses Gaia TGAS data to identify potential runaway and hypervelocity supergiant stars in the Large Magellanic Cloud, finding one candidate possibly escaping the galaxy and providing insights into stellar evolution and dynamics.

Contribution

First to analyze Gaia TGAS proper motions for LMC supergiants, identifying a candidate hypervelocity star and supporting binary evolution scenarios for R71.

Findings

Most stars follow the circular rotation model of the LMC.

R71 shows a moderate velocity deviation, supporting binary evolution origin.

One candidate hypervelocity star with ~360 km/s velocity, possibly escaping the LMC.

Abstract

We search for runaway stars in the Large Magellanic Cloud (LMC) by computing the space velocities of the visually brightest stars in the LMC that are included in the Gaia TGAS proper motion catalog. We compare with predictions from stellar dynamical models to obtain (peculiar) velocities relative to their local stellar environment. Two of the 31 stars have unusually high proper motions. Of the remaining 29 stars we find that most objects in this sample have velocities in very good agreement with model predictions of a circularly rotating disk model. Indeed the excellent fit to the model implies that the TGAS uncertainty estimates are likely overestimated. The fastest outliers in this subsample contain the LBV R71 and a few other well known emission line objects though in no case do we derive velocities consistent with fast (~100 km/s) runaways. Our results imply that R 71 in particular has a moderate deviation from the local stellar velocity field (40 km/s) lending support to the proposition that this object cannot have evolved as a normal single star since it lies too far from massive star forming complexes to have arrived at its current position during its lifetime. Our findings therefore strengthen the case for this LBV being the result of binary evolution. Of the two stars with unusually high proper motions we find that one, the isolated B1.5 Ia+ supergiant Sk-67 2 (HIP 22237), is a candidate hypervelocity star, the TGAS proper motion implying a very large peculiar transverse velocity (~360 km/s) directed radially away from the LMC centre. If confirmed, for example by Gaia Data Release 2, it would imply that this massive supergiant, on the periphery of the LMC, is leaving the galaxy where it will explode as a supernova.